This invention relates to an EDM power supply for generating self-adaptive discharge pulses and, more particularly, to improvements in power supply arrangements for the electric discharge machining of conductive workpieces.
In electrical discharge machining, hereinafter referred to as EDM, conductive workpieces are machined by passing electroerosive power pulses between a workpiece and a tool electrode spacedly juxtaposed therewith across an electrode gap flooded with a dielectric coolant which also serves to carry away the detritus of the electric discharge machining process.
In the EDM surfacing or cavity sinking of a conductive workpiece, a discrete electrical power pulse of a duration of 10.sup.1 7 to 10.sup.1 2 second may be applied across a relatively carefully dimensioned machining gap with a spacing, for example, of 0.05-0.005mm., to cause a spark discharge or a discharge of the short arc type to momentarily jump across the smallest dielectric path between a tool electrode and the workpiece, constituted as a counterelectrode. The applied electrical energy is highly concentrated (generally exceeding 10.sup.5 watts/cm..sup.2 with a current density of 10.sup.4 to 10.sup.9 amp/cm..sup.2) and is localized within the discharge column, thereby removing particles of that portion of the workpiece surface upon which the discharge impinges. As a consequence, a crater is formed in the workpiece surface opposite the electrode. The next time-spaced pulse may then seek another point of the work surface and bridge across the electrode and workpiece a further high-energy electroerosive discharge. A train of power pulses is thus formed to create localized material removal discharges which produce cumulatively overlapping craters in the workpiece surface; the total surface is thus machined uniformly over the parts thereof confronting the electrode and the machine portion receives a configuration conforming to the shape of the electrode.
The latter may be formed with the desired configuration of the cavity or the shape complementarily desired in the workpiece. During the machining operation, small metal or conductive chips or particles are carried away from the gap by the liquid dielectric which floods the gap and is generally circulated therethrough, while the tool electrode is advanced relative to the workpiece by a servo mechanism designed to maintain a predetermined gap spacing or designed to approach the desired gap spacing as accurately as possible.
In high-speed electric discharge machining operations, the energy of each individual discharge pulse is generally augmented for a given gap spacing to increase the amount of material removed per pulse; in addition or alternatively, the pulse repetition rate may be increased by reducing the discharge rest time or interval between one discharge pulse and the next to the minimum conistent with successive pulse formation and a stable cutting condition. In "no wear" operations, in which the tool electrode erosion is limited or eliminated, copper or graphite electrodes are customarily employed and are commonly poled positive while the workpiece electrode is poled negative, in contrast to normal machining operations in which the opposite polarity relationship in maintained. In such "no wear" operation, pulse duration must be relatively long, generally upwards of about 10 microseconds and the pulse amplitude or peak current must be controlled as not to exceed, say, 300 amperes. Excessively long pulses are avoided since they tend to produce discharges which transform an impulsive short period arc into a damaging thermal arc. Where increased fineness of the surface finish is required, a pulse train using narrower pulses is utilized. This latter type of pulse train results in a substantially reduced rate of removal of the workpiece material and also results in greater erosion of the tool electrode.